1. Field of the Invention
The present invention relates to a quartz crystal device having a configuration in which a quartz crystal blank is hermetically sealed in a container, and more particularly, to a quartz crystal device for surface mounting capable of normally operating even when the device receives a mechanical shock and maintaining a good vibration characteristic thereof.
2. Description of the Related Arts
Quartz crystal units having a quartz crystal blank hermetically sealed in a container, and quartz crystal oscillators, in which such a crystal unit and an IC (integrated circuit) chip having a circuit using the crystal unit are integrated, are generically called “quartz crystal devices.” The quartz crystal devices are used for a variety of electronic devices. For example, surface-mount crystal units having a crystal blank hermetically encapsulated in a container are small and light, and are therefore incorporated together with an oscillation circuit in portable electronic devices represented by cellular phones as frequency and time reference sources.
In recent years, these quartz crystal devices are increasingly made more compact. In the case of a surface-mount crystal unit, standardization whereby the planar outer dimensions of a container are defined as 5×3.2 mm or 3.2×2.5 mm is underway and various types of crystal blanks are accommodated in these containers according to their applications and functions. Moreover, crystal units having much smaller dimensions than these standardized dimensions are also put to practical use. Such downsized quartz crystal devices are required to maintain their vibration characteristics when a mechanical shock is applied thereto and prevent frequency variations in particular.
FIG. 1A is a cross-sectional view showing an example of configuration of a conventional surface-mount crystal unit, and FIG. 1B is a plan view of the crystal unit shown in FIG. 1A with the cover removed.
The illustrated surface-mount crystal unit is made up of crystal blank 2 accommodated in container body 1 for surface mounting, covered with metal cover 5 and hermetically sealed in the container. Container body 1 is made of, for example, laminated ceramics and has a substantially rectangular outside shape, that is, a substantially flat rectangular parallelepiped shape having a rectangular top view when this crystal unit is mounted on a wiring board. A recess is formed in the top surface of container body 1 to accommodate crystal blank 2. A pair of holding terminals 3 are provided on an inner bottom surface of the recess near both ends of one side of the inner bottom surface. Holding terminals 3 are used to electrically and mechanically holding crystal blank 2 in the recess. Metal cover 5 is bonded to the top surface of container body 1 by means of seam welding to close the recess thereby hermetically sealing crystal blank 2 in the recess.
Container body 1 is provided with outside terminals 4 in the four corners of the outer bottom surface of container body 1, that is, the surface facing a wiring board when mounted on the wiring board. Outside terminals 4 are used to surface-mount container body 1 on the wiring board. Each outside terminal 4 is formed as a substantially rectangular conductive layer. Of these four outside terminals 4, a pair of outside terminals 4 located at both ends of one diagonal on the outer bottom surface of container body 1 are electrically connected to a pair of holding terminals 3 via a conductive path formed in the lamination plane between the ceramic layers of container body 1. Furthermore, remaining two outside terminals 4 are used as grounding terminals. Outside terminals 4 used as the grounding terminals are electrically connected to metal cover 5 via a conductive path (not shown) formed in container body 1.
Crystal blank 2 is made up of, for example, a substantially rectangular AT-cut quartz crystal blank as shown in FIG. 2 Excitation electrodes 6a are formed on both principal surfaces of crystal blank 2. The positions where excitation electrodes 6a are formed constitute vibration regions of crystal blank 2. Lead-out electrodes 6b extend from the pair of excitation electrodes 6a toward both sides of one end of crystal blank 2. At the position of the end of crystal blank 2, lead-out electrodes 6b are formed so as to fold back between both principal surfaces of crystal blank 2. Crystal blank 2 is fixed in the recess of container body 1 by securing these lead-out electrodes 6b to holding terminals 3 at the positions where the pair of lead-out electrodes 6b are led out using, for example, conductive adhesive 7 or the like and electrically and mechanically connected to container body 1.
Conductive adhesive 7 is, for example, of a thermosetting type and coated on holding terminals 3 as a primer coat beforehand. When crystal blank 2 is fixed, both sides of one end of crystal blank 2 are placed on conductive adhesive 7 and conductive adhesive 7 is then heat set. Alternatively, it is also possible to apply the conductive adhesive onto holding terminals 3 as a primer coat, place both sides of one end of crystal blank 2 on conductive adhesive 7, further apply conductive adhesive 7 on the top surface on both sides of one end of crystal blank 2 as a topcoat and then heat-set conductive adhesive 7.
As is well known, the AT-cut crystal blank operates in a thickness-shear vibration mode and has a vibration frequency inversely proportional to the thickness thereof. Examples of the cross-sectional shape along the longitudinal direction of crystal blank 2 include a bevel shape, convex shape and flat shape. As shown in FIG. 3A, a bevel-shaped crystal blank has a thickness which is constant over a certain range of breadth of the central part and decreasing from the central part toward the periphery. As shown in FIG. 3B, a convex-shaped crystal blank has a gently varying thickness which becomes a maximum at the center of the crystal blank. As shown in FIG. 3C, a flat-shaped crystal blank has a constant thickness over the entire range. When the vibration frequency is approximately 30 MHz or more, crystal blank 2 is formed into a flat shape. On the other hand, when the vibration frequency is lower than 30 MHz, crystal blank 2 is formed into a bevel shape or convex shape through edge dressing to confine the vibration energy within the central region of crystal blank 2 and reduce crystal impedance (CI) of crystal blank 2.
Of both ends in the longitudinal direction of crystal blank 2, one end which is fixed to container body 1 by conductive adhesive 7 is called a “fixed end” and the other end is called a “free end.” Pillow member 8 protruding from the inner bottom surface of container body 1 is provided on the inner bottom surface of container body 1 at a position corresponding to the free end of crystal blank 2. The free end of crystal blank 2 is placed on pillow member 8 without being fixed to pillow member 8. The free end may also be placed above pillow member 8 so as not to contact pillow member 8. Provision of the pillow member is disclosed, for example, in Japanese Patent Laid-Open Application No. 2001-237665 (JP-A-2001-237665) and Japanese Patent Laid-Open Application No. 2003-32068 (JP-A-2003-032068). When the cross-sectional shape in the longitudinal direction of crystal blank 2 is assumed to be a bevel shape or convex shape as described above, pillow member 8 is intended to prevent the vibration region in which particularly excitation electrode 6a of crystal blank 2 is formed from contacting the inner bottom surface of container body 1. Also in the case where the cross-sectional shape of crystal blank 2 is a flat shape, the vibration region of crystal blank 2 may contact the inner bottom surface of container body 1 due to warpage or the like of container body 1, and therefore pillow member 8 is effective in such a case, too.
Such pillow member 8 is provided simultaneously with a tungsten (W) layer or molybdenum (Mo) layer formed as an underlying electrode layer making up holding terminals 3 using a printing method when ceramic green sheets are laminated and burned to form container body 1. Alternatively, pillow member 8 may be made of ceramics, and integrally burned and formed with container body 1.
Pillow member 8 reduces the swinging range of the free end of crystal blank 2 when a mechanical shock is applied to the quartz crystal device and maintains the vibration characteristic of crystal blank 2 satisfactorily. Upon receiving a shock, crystal blank 2 swings around the fixed end as the axis, but the provision of pillow member 8 reduces the swinging range at the free end, and therefore the degree of swinging of crystal blank 2 also decreases and influences on conductive adhesive 7 which holds the crystal blank at the fixed end also decrease. The vibration system of crystal blank 2 including conductive adhesive 7 has less variation by shock, and can thereby maintain the vibration characteristic satisfactorily.
On the other hand, when pillow member 8 is not provided, the swinging range on the free end side of crystal blank 2 increases when a shock is applied, the influence of the swinging also extends to conductive adhesive 7, causes a variation in the state thereof, that is, the influence reaches the vibration system and deteriorates the vibration characteristic of the crystal blank. In this way, pillow member 8 provided for the free end of crystal blank 2 is meaningful in two aspects: preventing the vibration region of crystal blank 2 from contacting the inner bottom surface of container body 1; and maintaining the vibration characteristic of the vibration system against shocks.
As described above, since the thickness of crystal blank 2 is inversely proportional to the vibration frequency, the higher the vibration frequency, the smaller the thickness of crystal blank 2 becomes and the planar outside size of crystal blank 2 also reduces accordingly. On the contrary, the lower the vibration frequency, the greater the thickness of crystal blank 2 becomes and the planar outside size increases. In the case of volume production of crystal blanks, edge dressing by which the shape in the longitudinal direction of the crystal blank is formed into a bevel shape or convex shape is performed by generally putting many crystal blanks into a cylindrical or spherical hollow recipient together with abrasives and rotating the hollow recipient. As the hollow recipient rotates, the perimeter of crystal blank 2 is worked into a curved surface along the inner perimeter of the hollow recipient. Through such work, an inclined surface having a curved surface is formed on the crystal blank not only in the longitudinal direction but also in a direction parallel to the short side of crystal blank 2.
In this way, when the cross section in the short side direction of crystal blank 2 is thick in the central part and thin at both ends, it is necessary to make holding terminals 3 thicker and increase the height of holding terminals 3 from the inner bottom surface to prevent crystal blank 2 from contacting the inner bottom surface of container body 1. Therefore, two coats of the underlying electrode layer made of tungsten, molybdenum or the like are given to holding terminals 3 by printing as shown in FIG. 4. Of the underlying electrode layer, a layer directly contacting the inner bottom surface of container body 1, that is, a bottom layer is assumed to be first layer 3a and a layer formed on first layer 3a is assumed to be second layer 3b. In this way, the underlying electrode layer formed using the printing method is integrally burned and formed with container body 1, then nickel (Ni) and gold (Au) plating is applied to the surface of the underlying electrode layer. Since holding terminals 3 are formed by printing, the area of a higher layer is reduced by an amount corresponding to the width of the opening of a mask used for printing. First layer 3a not only functions as holding terminals 3 but also is used for a circuit pattern to connect holding terminals 3 and outside terminals 4, and therefore the thickness of first layer 3a is reduced and the thickness of second layer 3b is increased.
When both sides of one end of crystal blank 2 are secured to holding terminals 3, this prevents the central part in the short side direction of the crystal blank having a relatively large thickness from contacting the inner bottom surface of container body 1.
The width, that is, length in the short side direction, of the crystal blank is made to change for each vibration frequency band of the crystal blank to avoid spurious vibration according to the width. Therefore, the length of each holding terminal 3 along the short side direction of the crystal blank is increased to support various types of crystal blank 2 of different vibration frequency bands using the same container body 1. This allows various types of crystal blank 2 in different planar outside sizes, especially different width dimensions to be connected to a pair of holding terminals 3 while standardizing container body 1, and improves productivity.
However, the above described conventional surface-mount crystal unit has a problem that as the miniaturization advances, the vibration frequency changes when a shock is applied.
When the planar outside size of crystal blank 2 reduces as the miniaturization of the crystal unit advances, the vibration characteristic, especially the crystal impedance (CI) deteriorates if no modification is made, and therefore the area of excitation electrode 6a is generally increased. When, for example, the planar outside size of crystal blank 2 is assumed to be 2.1×1.45 mm, excitation electrode 6a is formed by using vapor deposition or a sputtering method on substantially the entire surface of each principal surface of crystal blank 2 except a portion of 0.1 to 0.15 mm corresponding to the frame width of the mask. However, since holding by conductive adhesive 7 is taken into consideration at the position where lead-out electrode 6b is led out, excitation electrode 6a is not formed to the limit of the perimeter of the crystal blank. Therefore, excitation electrode 6a is formed on the three sides of substantially rectangular crystal blank 2 except the electrode-free portion of 0.1 to 0.15 mm from the outer edge. As a result, the outer dimension of excitation electrode 6a becomes on the order of 1.6×1.2 mm and the area thereof becomes approximately 63% of the area of crystal blank 2. When excitation electrode 6a is formed so as to cover most part of the principal surface of crystal blank 2 in this way, if the free end of crystal blank 2 contacts pillow member 8 due to a shock, not only the electrode-free portion at the tip of the free end but also excitation electrode 6a contacts pillow member 8. When the present inventors observed excitation electrode 6a of a portion which seemed to have contacted pillow member 8, damage was found in excitation electrode 6a of that portion. The present inventors guessed that this damage would be the cause for the vibration frequency variation.
Moreover, though in the above described surface-mount crystal unit, container body 1 is standardized for various types of crystal blank 2 having different widths, when a shock is applied, crystal blank 2 is liable to peel off holding terminals 3. That is, the above described crystal unit has an insufficient anti-shock characteristic. Especially, the crystal blank subjected to edge dressing that forms its cross section into a bevel shape or convex shape has a tendency to have a small fixing strength with respect to the holding terminals.
When crystal blank 2 subjected to edge dressing into a bevel shape or convex shape and having a large width is secured to holding terminals 3, the inclined surface of crystal blank 2 contacts the mutually facing edges of the pair of holding terminals 3 as shown in FIG. 4. Here, since holding terminals 3 are also used to fix crystal blank 2 of a small width, distance W1 between holding terminals 3 is short. Therefore, the position where crystal blank 2 contacts holding terminals 3 is a position near the center of crystal blank 2 and the end in the width direction of crystal blank 2 is located above and apart from the surface of holding terminals 3 as indicated by separation distance d1.
In contrast to this, conductive adhesive 7 fixes crystal blank 2 to holding terminals 3 at positions corresponding to both ends in the width direction of crystal blank 2 to avoid influences on the characteristic of crystal blank 2 in the vibration region. Even when conductive adhesive 7 is applied to holding terminals 3 so as to correspond to both ends in the width direction of crystal blank 2, crystal blank 2 is placed on conductive adhesive 7 and crystal blank 2 is pressed, the surface of crystal blank 2 contacts the edges of holding terminals 3, and therefore no pressing force is transmitted to conductive adhesive 7. Therefore, the thickness of conductive adhesive 7 for fixing crystal blank 2 to holding terminals 3 increases especially in the portion contacting the periphery of the crystal blank, conductive adhesive 7 applied to holding terminals 3 does not spread, and the area of contact between holding terminals 3 and conductive adhesive 7 and the area of contact between conductive adhesive 7 and crystal blank 2 become small. The fixing strength of crystal blank 2 by conductive adhesive 7 with respect to holding terminals 3 decreases. In this case, even the amount of conductive adhesive 7 is increased to expand the contact area, the thickness of conductive adhesive 7 does not decrease, and therefore the fixing strength against shocks does not substantially improve. Further increasing the amount of conductive adhesive 7 causes conductive adhesive 7 to adhere even to the position near the center of crystal blank 2, deteriorating the vibration characteristic of crystal blank 2, producing variations in the spread of conductive adhesive 7, thus making the vibration characteristic non-uniform.
After all, when crystal blank 2 having an inclined surface through edge dressing is fixed to holding terminals 3 of container body 1, which is standardized for various types of crystal blank 2 having different planar outside shapes, the anti-shock characteristic deteriorates.